1. Field of the Invention
The present invention relates to a sheet processing device and an image forming system, more particularly to sheet folding.
2. Description of the Related Art
In recent years, digitalization of information has been promoted, and image processing apparatuses, such as printers or facsimile machines used for output of digitalized information, or scanners used for digitalization of documents, are essential to the digitalization of information. Such image processing apparatuses each include an imaging function, an image forming function, a communication function, and the like, and are often provided as multifunction peripherals each used as a printer, a facsimile machine, a scanner, and a copying machine.
Among such multifunction peripherals, a multifunction peripheral is known which is mounted with a folding device for forming an image on a fed sheet to draw an image, and then folding the sheet on which the image has been formed. When such a folding device folds a sheet to make a fold line, the fold line is not so firm and incomplete, and the fold line has a high folded height.
Therefore, among such multifunction peripherals, a multifunction peripheral is known which is mounted with an additional folding device performing additional folding for securing a fold line, by pressing the fold line formed by folding a sheet to secure a fold line, and reducing the height of the fold line, in addition to the folding device.
When such a folding device as described above folds a sheet, the fold line is generally formed in a direction (hereinafter, also referred to as a “direction perpendicular to a sheet conveying direction”) perpendicular to a direction in which the sheet is conveyed (hereinafter, also referred to as a “sheet conveying direction”).
Therefore, an additional folding method for such an additional folding device as described above includes, for example, a method in which an additional folding roller is laterally bridged in a direction parallel to a fold line formed by folding a sheet (in a direction perpendicular to a sheet conveying direction), the additional folding roller is rotated about a rotation axis extending in the direction perpendicular to the sheet conveying direction, and pressing a fold line formed in a sheet while conveying the sheet (e.g., see Japanese Laid-open Patent Publication No. 2007-045531).
Further, another additional folding method for such an additional folding device as described above includes, for example, a method in which conveyance of a sheet is once stopped at a position where additional folding is performed, an additional folding roller rotated about a rotation axis extending in a direction (sheet conveying direction) perpendicular to a fold line formed by folding a sheet is moved in a direction perpendicular to the sheet conveying direction while being pressed against the stopped sheet, and sequentially presses the fold line formed in the sheet in the direction perpendicular to the sheet conveying direction (e.g., see Japanese Laid-open Patent Publication No. 2009-149435).
The additional folding method of Japanese Laid-open Patent Publication No. 2007-045531 requires a plurality of additional folding rollers in the sheet conveying direction. It is because one additional folding roller presses the whole area of the fold line simultaneously, a pressing force of the one additional folding roller is dispersed over the whole area of the fold line, a pressing force per unit area is reduced, and only the one additional folding roller cannot bring about sufficient effect of additional folding. Accordingly, when such a method is used to perform the additional folding, a space for disposition of the plurality of additional folding rollers is required, a multifunction peripheral is increased in size, a drive system or a control system needs to be added to drive the additional folding rollers, and an initial cost and a running cost are disadvantageously increased.
Meanwhile, in the additional folding method of Japanese Laid-open Patent Publication No. 2009-149435, the whole area of the fold line is sequentially pressed by one additional folding roller in the direction perpendicular to the sheet conveying direction, so that a concentrated pressing force can be applied to the whole area of the fold line portion, and the pressing force is prevented from being dispersed, but, during additional folding, the additional folding roller needs to be moved from one end to the other end in a sheet width direction while the sheet is stopped. Accordingly, when such a method is used to perform the additional folding, a time is required for movement of the additional folding roller from one end to the other end in the sheet width direction, and productivity is disadvantageously reduced.
Therefore, a method may be provided in which an additional folding roller is laterally bridged in a direction perpendicular to the sheet conveying direction, having a surface formed with a pressing member having a helical shape about the rotation axis, and rotated about a rotation axis extending in a direction perpendicular to the sheet conveying direction, and when the additional folding roller is rotated, a fold line formed in a sheet in a direction perpendicular to the sheet conveying direction is sequentially pressed. According to such an additional folding device, only part of the helical pressing member formed on the surface of the additional folding roller makes contact with the sheet, so that the additional folding roller is rotated to sequentially press the fold line formed in the sheet in a direction perpendicular to the sheet conveying direction.
Accordingly, such an additional folding device allows one additional folding roller to apply a concentrated pressing force to the whole area of the fold line for a short time, and a sufficient pressing force can be applied to the fold line at low cost without reducing productivity.
However, in such an additional folding device, when the pressing member formed on the surface of the additional folding roller abuts on the sheet, the concentrated pressing force is rapidly applied to the abutment part, impact sound is generated, and a noise disadvantageously occurs outside the device.
In view of the above-described conventional problem, there is a need to efficiently press a fold line formed in a sheet at low cost, and to reduce a noise generated upon pressing the fold line.